Maximum withstandability torque for a shaft

Hi

Refer the below images:







Green part name: Manual override

Function of the manual override:

Rotate around its axis and the rotation stopped with the flat face at one end.

When applying torque at screw driver slot, the manual override starts rotate.

Rotation stops when the flat faces are touched each other.

If I am continue the torque / increase the torque, I need to find out the maximum withstandability torque.

Shear stress, polar moment of inertia, Twist / torsional stress.

Kindly provide your feedback.

Thanks with Regards.